1. Field of the Invention
The present invention relates to wheat starch having novel properties and, more particularly, to wheat starch having a high apparent amylose content.
2. Description of the Related Art
Starch is the major component of the endosperm of a cereal seed such as wheat. Wheat starch components can be either amylose or amylopectin. The amylose content of wheat starch is 0% for waxy wheat cultivars and about 22-30% (about 29% on average) for normal (non-waxy) wheat cultivars.
Some maize cultivars yield corn starch in which the amylose content is as high as about 60-70%. Corn starch with high amylose content provides various industrial applications, such as an adhesive for cardboard, a converging agent for glass fiber and an edible film, as well as food applications such as ricemeal which is used when making rice cake.
Among various rice varieties, Indica rice grain has a higher amylose content than Japonica rice grain. Rice grains having a high amylose content can be suitably used for pilaf and rice vermicelli.
Wheat starch with a high amylose content and wheat flour containing such wheat starch are expected to provide new industrial and food applications. Therefore, attempts have been made to produce wheat starch with increased amylose content using crossbreeding and genetic engineering approaches. However, to the extent the present inventor is aware of, no satisfactory results have been obtained.
Amylose is an α(1,4)-linked glucose polymer which is essentially a linear chain without branching. Amylopectin is a branched glucose polymer in which branch chains are linked to the main chain of α(1,4)-linked polymer by α(1,6)-linkages. The linear glucose polymers are synthesized by the action of starch synthases which produces (1,4)-linkages. The (1,6)-linkages of amylopectin are produced by the action of branching enzymes.
Studies in pea, maize, and wheat (Denyer et al., Plant J. 4:191-198, 1993; Echt and Schwartz, Genetics 99:275-284, 1981; Mu et al., Plant J. 6:151-159, 1994; and Denyer et al., Planta 196:256-265, 1995) have shown that some enzymes for starch synthesis are tightly bound to starch granules from seed endosperms of maize and wheat and pea embryo.
The detailed mechanism for the binding of these enzymes to starch granules has been unknown. However, it is believed that in wheat, at least four kinds of proteins, i.e., waxy protein and three starch granule proteins (SGP-1, SGP-2, SGP-3), are tightly bound to starch granules and are responsible for starch synthesis. Waxy protein, i.e., granule-bound starch synthase I (GBSS I) responsible for amylose synthesis, is the product of the waxy gene (Ainsworth et al., Plant Mol Biol. 22:67-82, 1993). SGP-1, -2 and -3 (Yamamori and Endo, Theor Appl. Genet. 93:275-281, 1996) correspond to starch granule-bound isozymes of about 100-105 kDa, about 90 kDa and about 77 kDa , respectively, reported by Denyer et al. (Planta, supra). Immunoblotting, amino acid sequencing and detection of starch synthase or branching activities (Denyer et al., Planta, supra; Rahman et al., Aust. J. Plant Physiol. 22:793-803, 1995; Takaoka et al., J. Agric. Food Chem. 45:2929-2934, 1997) suggest that SGP-2 is a homolog of maize branching enzyme IIb (Fisher et al., Plant Physiol. 102:1045-1046, 1993) and that SGP-3 is a homolog of maize starch synthase I (Knight et al., Plant J. 14:613-622, 1998).
Immunoblotting studies on about 100-105 kDa protein (SGP-1) did not detect the protein in the soluble fraction. Thus, SGP-1 is exclusively bound to starch granules (Denyer et al., Planta, supra; Rahman et al., supra). This protein is presumed to be a starch synthase from the studies of antiserum recognition, enzymatic activity detected and homology in amino acid sequences (Denyer et al., Planta, supra; Takaoka et al., supra). However, information regarding the physiological function of SGP-1 in vivo has been limited. For maize, it has been reported that an apparent amylose content is increased in a mutant of dull 1 gene which is presumed to code for starch synthase II (Gao et al., The Plant Cell 10:399-412, 1998). However, there is no substantial amino acid sequence homology between the protein coded by dull 1 (Gao et al., supra) and the protein SGP-1 of wheat (Takaoka et al., supra). Further, the protein coded by dull 1 is present in the soluble fraction. Thus, the starch synthase encoded by dull 1 is significantly different from SGP-1.
A hexaploid wheat has three isozymes of SGP-1, i.e., SGP-A1, SGP-B1 and SGP-D1. The gene coding for SGP-A1, Sgp-A1, is located on chromosome arm 7A, Sgp-B1 on 7B, and Sgp-D1 on 7D (Denyer et al., Planta, supra). Using SDS-polyacrylamide gel electrophoresis (SDS-PAGE), it has been found that a few wheat cultivars lacked either SGP-A1, -B1 or -D1, but no wheat cultivars lacked two or more SGP-1s (Yamamori and Endo, supra).